1. Field of the Invention
This invention relates to a membrane switch and a pressure sensitive sensor, which are composed by bonding a pair of electrode sheets and a spacer interposed therebetween with an adhesive interposed therebetween.
2. Description of the Related Art
The membrane switch is a laminated structure composed of a pair of electrode sheets having electrodes formed respectively on opposite surfaces of a pair of sheetlike base materials disposed oppositely, the electrodes constituting a contact portion, and of a sheetlike spacer interposed therebetween. In order to allow the upper and lower electrodes to come into contact with each other by an appropriate pressing force, the contact portion of the membrane switch is configured in such a manner that a hole of a predetermined size is drilled in the spacer, and the upper and lower electrodes come into contact with each other through this hole. Moreover, as a similar configuration to the membrane switch, a pressure sensitive sensor using pressure sensitive ink has been known. In this pressure sensitive sensor, at least one of the opposite electrodes has been a pressure sensitive electrode made of the pressure sensitive ink so that a resistance value can be changed according to a pressure applied thereto.
The electrode sheets and the spacer are adhered by the adhesive. As such a spacer, it is general to use a spacer with adhesive (double coated adhesive sheet with base material), in which the adhesive is applied on both surfaces of the spacer beforehand. Moreover, in some cases, a spacer having an adhesive paste printed thereon by use of a printing technology is used. In the case of using the spacer with adhesive, a level spacer sheet is subjected to drilling processing by use of a die or the like. Accordingly, the adhesive is provided to a peripheral portion of the hole formed in the spacer. Various types of spacers and adhesives are used in accordance with a purpose of a product and an affinity of the material therewith. As general spacers, PET, PEN, PEI, PI and the like are used. Moreover, as representative adhesives, ones of an acrylic series, a urethane series, a silicone series and the like are given.
In the case of the membrane switch and the pressure sensitive sensor as described above, an interval between the upper and lower electrodes, a hole diameter of the spacer, a rigidity of the upper and lower electrode sheets, a viscoelastic property of the adhesive and the like mainly become parameters for deciding a load necessary to bring the upper and lower electrode sheets into contact with each other. Accordingly, in order to conduct electricity through the point of contact by means of a desired pressure or force, it is necessary to set these parameters at appropriate values.
Moreover, it is also important for sensitivities of the membrane switch and the pressure sensitive sensor not to vary very much depending on a temperature environment. Temperature dependency of the sensitivities of the membrane switch and the pressure sensitive sensor is determined by a temperature property of the above-described parameters. Among these, since the interval between the upper and lower electrodes and the hole diameter of the spacer hardly vary, they have little relationship with the sensitivities. However, the rigidity of the electrode sheets and the viscoelasticity of the adhesive have temperature dependency and affect the sensitivities largely.
When the upper and lower electrodes are warped, an adhesive layer is deformed accompanying this. The rigidity of the electrode sheets is derived from temperature dependency of an elastic modulus proper to the electrode sheet material, and is determined by a selected material and processing conditions thereof. A material having smaller temperature dependency of the elastic modulus will have smaller temperature dependency of the rigidity. The temperature dependency of the adhesive is also similar to the above, that is, this temperature dependency is a physical property proper to the material, and an adhesive having small temperature dependency is required.
Accordingly, in order to improve the temperature property in terms of the structure while leaving the structure as it is, there is no other way but to select a material. However, when a material having small temperature dependency is selected, cost is increased, and therefore, there has been no effective means for improving the temperature property in terms of the structure at low cost.
This invention was made in order to solve the subjects as described above. The object of the present invention is to provide a membrane switch and a pressure sensitive sensor, which are capable of improving the temperature property in terms of the structure at low cost.
A membrane switch according to the present invention includes: a pair of electrode sheets having electrodes formed respectively on opposite surfaces of a pair of sheetlike base materials disposed oppositely, the electrodes constituting a contact portion; a spacer interposed between the pair of electrode sheets so that the electrode sheets are opposed to each other with a predetermined interval spaced therebetween, the spacer having a hole formed in a position of the contact portion; and an adhesive for bonding the spacer between the pair of electrode sheets, wherein the adhesive on at least one surface side of a peripheral portion of the hole of the spacer is removed.
A pressure sensitive sensor according to the present invention includes: a pair of electrode sheets having electrodes formed respectively on opposite surfaces of a pair of sheetlike base materials disposed oppositely, the electrodes constituting a contact portion, and at least one of the electrodes being a pressure sensitive electrode; a spacer interposed between the pair of electrode sheets so that the electrode sheets are opposed to each other with a predetermined interval spaced therebetween, the spacer having a hole formed in a position of the contact portion; and an adhesive for bonding the spacer between the pair of electrode sheets, wherein the adhesive on at least one surface side of a peripheral portion of the hole of the spacer is removed.
According to the present invention, there are removed the adhesives on both surfaces (used to deform the electrode sheets on the both surfaces) of the peripheral portion of the spacer hole for bringing the upper and lower electrodes into contact with each other or on one surface thereof (used to deform the electrode sheet on the one surface). Therefore, when the adhesive is deformed accompanying the deformation of the electrode sheet, even if an deformation amount is changed due to temperature change (for example, even if the adhesive is hard to be deformed at low temperature and apt to be deformed at high temperature), the deformation of the electrode sheet becomes a deformation with a contact point or contact line of the electrode sheet and the spacer as a fulcrum from a point of time when the electrode sheet and the spacer contact with each other. Therefore, the deformation comes hardly to be affected by the viscoelastic property of the adhesive. Thus, it is made possible to improve the temperature property in terms of the structure, which is derived from the viscoelasticity of the adhesive.
With regard to a method for removing or retreating the adhesive from the peripheral portion of the spacer hole, various methods are conceivable. For example, in the case of forming the adhesive by printing, a region on a printing pattern, where the adhesive (adhesive paste or the like) is not printed, is made larger than a diameter of the spacer hole. In the case of using a transcription type adhesive, it is recommended to drill a hole larger than the spacer beforehand by drilling processing for a transcription sheet.
Moreover, if a convex portion is previously formed of a material having small temperature dependency of the viscoelasticity than the adhesive, and preferably, of a material having temperature dependency of the elastic modulus as small as or smaller than that of the electrode sheet on the opposite surface of the electrode sheet opposite to the peripheral portion of the spacer hole, from which the adhesive is removed, then the convex portion and the spacer contact with each other at an earlier stage after the electrode sheet starts to be warped. Consequently, it is made possible to eliminate the influence of the elastic modulus of the adhesive earlier than the case of not providing the convex portion and to improve the temperature dependency further.
It is desirable that the convex portion be formed of the same material in the same process as those of the electrode. Particularly, in the case where the electrode or the pressure sensitive electrode is formed of conductive paste or pressure sensitive ink by a method such as screen printing, if the convex portion is formed of the same material as that of the electrode, then the convex portion can be also formed in the process of forming the electrode by printing. Therefore, the reduction in manufacturing cost can be achieved. In the case where the convex portion is formed of the same material in the same process as those of the electrode, as compared with the case where the convex portion is formed of another material, effects can be expected, in which the interval between the upper and lower electrodes is held constant, and sensitivity change with respect to pressure necessary to contact the upper and lower electrodes each other can be reduced. This is because, in the case of forming the convex portion and the electrode of the same material, at the point of time when the upper and lower electrode sheets start to be warped and the convex portion and the spacer contact with each other, the interval between the electrodes always becomes equal to a thickness of the spacer. Thus, in the case of multi-contact switching unit and pressure sensitive sensor, variations in sensitivity in terms of the structure can be suppressed to be smaller.